Process of making rustless iron alloys



Feb. 17', 1931. F. M. BECKET ET AL PROCESSfOF MAKING RUSTLESS IRONALLOYS Filed July 20. 1927 0 0X en 1' portions replacing the iron.

i atented Feb. 17, 1931 I UNITED STATES PATENT OFFICE FREDERICK H.BECKET, OF NEW YORK, AND JAMES H. OBITOHETT, OF DOUGLASTON, NEW YORK,ABSIGNORS TO ELECTBO METALLUBGIOAL'OQ, A CORPORATION OF WEST VIRGINIA.

PROCESS OF MAKING RUSTLESS IRON ALLOYS Application'med m 20, 1927.Serial No. 207,288.

This invention relates to themanufacture of low-carbon iron-chromiumalloys, and particularly to those alloys of the so-called rustless irontype. Rustless iron, for the :5 purposes of this invention, may bedefined as an alloy consisting essentially of iron and chromium, withupward of about 9% of chromium and less than 0.2% of carbon. Nickel mayalso be present in varying pro- A preferred composition contains about12-14% of chromium and 0.12% or less of carbons. Another excellentcomposition contains around 1618% of chromium, 78% of nickel, and

around 0.12% or less of carbon.

Rustless irons have generally been produced by direct addition oflow-carbon ferrochromium to a low-carbon iron bath or, alternatively, toa very limited extent by reducing chromium into such iron bath by meansof a non-carbonaceous reducing agent, such as aluminum or silicon.Low-carbon ferro-chromium is somewhat costly and to produce the chromiumcontent desired in the rustless irons according to the alternativemethods is even more expensive. High-carbon ferrochromiums, i. e. thosecontaining from 4 to 7% of carbon on the other hand ofier a relativelycheap source of chromium.

Edmands in U. S. Patent No. 1,063,341

and Morehead in U. S. Patent 1,063,285 have su gested Bessemerization asa method for reducing the carbon content of high-carbon ferrochromium.Our experiments have shown that when Bessemerizing with air it isimpossible to obtain an'alloy of a carbon content below about 2.0% or achromium to carbon ratio of above 35 without causing serious losses inchromium recovery. When using enriched air or oxygen in such processes,the carbon content has been reduced to a greater extent, but when anattempt has been made to Bessemerize to such an extent that the chromiumto carbon ratio inthe product is greater than about 70 the loss ofchromium has become practically prohibitive.

Qur investigations have proven that by is correspondingly short.Nevertheless, we

chromium content corresponding rustless lrons it is possible to obtain amuch higher when Bessemerizing ferrochromium. Thus, by direct action ofoxygen-enriched air, or preferably commercially pure oxygen, applied tothe molten alloy in a side blown converter, such lower chromium rustlessirons may be commercially reduced in carbon content below about 0.2% andeven as low as about 0.02%, or to such an extent as to reach a chromiumto carbon ratio of from 75 to 500. In the manufacture of rustless ironparticularly, we have thus realized decided economies, among which are amuch greater speed of decarburization and a materially hlgher ratio ofchromium to carbon in the pgpduct than have been heretofore attaina e.

The preferred conditions, as disclosed by our investigations, are asfollows:

1. The oxidizin blast should be decidedly richer in oxygen t anatmospheric air, is desirably above oxygen, and preferably isapproximately pure oxygen. nder these conditions the oxidation ofcarbon, chromium, silicon, and any other readily oxidizable componentsof the melt is extremely energetic, and the time required for the blowhave demonstrated that under these conditions there is a certaindefinite selective tendency toward the oxidation of carbon, with theresult that, for a given low carbon content, decidedly better recoveriesof chromium are obtained as the oxygen concentration of th blastincreases.

This is illustrated by the accompanying graph, in which the curve Arepresents chromium recoveries, from iron-chromium alloys containingchromium, suitable for rustchromium to carbon ratio than is possibleless iron and initially about 1.5% (avera of four heats) of carbon, at aperiod of t e treatment when about 96-98% of the contained carbon hadbeen oxidized, as indicated by curve B. As appears from this graph therecovery of chromium usin atmospheric air (21%- 0 gen) was only a out23%, under the speci 0 test conditions; with 30% oxygen the recovery was42%; with 60% oxygen 68%; and with 100% oxygen nearly 79%. It will ofcourse be understood that these figures apply only to the testconditions, temperatures, scale of operation, etc., as actuallyemployed, and are not intended to represent the maximum recoveriesattainable in commercial practice. The graph very clearly indicateshowever the advantage of employing a blast having an oxygen content ofat least and as near 100% as is available for use.

2. The blowing is carried out in a converter of the so-called side-blowntype, in which the blast is introduced above and applied to the surfaceof the molten metal, or in an essentially equivalent apparatus.

Where atmospheric air is used for the blowing, broadly speaking, it isimmaterial whether a side-blown or bottom-blown converter is employed,although neither of these is effective to oxidize the carbon withoutexcessive losses of chromium. On the other hand, when highconcentrations of oxygen are used as contemplated herein, we have foundit undesirable, commercially, to employ a converter of the bottom-blowntype on account of the rapid destruction of the bot tom lining at thetemperatures created by the exothermic reactions; but with a converterof the side blown type linings of sufficiently refractory character havea satisfactory life. The best results have been obtained with a liningof chromium ore or a lining of magnesia, previously fused in theelectric furnace.

Accordingly, our invention contemplates broadly the Bessemerization ofiron-chromium alloys containing not more than about 2% carbon, to reducethe carbon content thereof below about 0.2% and in some instances as lowas about 0.02%; and, more specifically, contemplates economicallytreating such alloys, so as to avoid prohibitive chromium losses, byusing an oxidizing blast containing a substantially higher percentage ofoxygen than is present in atmospheric air, and applying such blast inaside-blown converter or equivalent apparatus in which the blast isapplied to the surface of the molten charge. So far as we are aware suchconditions have not heretofore been soapplied in the treatment of thesealloys as to obtain the results stated.

Following are illustrative examples of procedure in accordance with ourinvention, but it is to be understood that the invention is not limitedto the precise details described in such examples.

Steel scrap and high-carbon ferrochromium, with nickel or otherappropriate metal if desired, in the proportions required to give thedesired chromium content (taking the losses into consideraton) aremelted in an electric or other suitable furnace to a highly.

fluid condition. The molten charge is quickly transferred to aside-blown converter which is then turned up to its operating position,whereupon a blast of oxygen or oxygen-enriched air is applied to thesurface of the bath and adjusted until the flame from the converterindicates vigorous action. At first a very short flame issues from theconverter; after a short blow this flame lengthens and becomesconsistently brighter. The elimination of carbon is accompanied by anapparent boiling of the molten alloy, is very rapid, and the flame soondrops. The heat is blown for only a brief period, the duration dependingupon the oxygen content of the blast and the carbon content desiredinthe product. Then the metal is poured into a ladle, the usualdeoxidizers are added, and the metal discharged into ingotmoulds. Thisfinal deoxidation may be accomplished in other ways, e. g., in anelectric furnace where the operation is finished.

Alternatively, high-carbon chromium steel may be provied by smeltinganappropriate mixture of chromium ore and iron ore or metallic iron,such as steel scrap, so as to produce an alloy of chromium contentsubstantially suitable for rustless iron but containing an excessive percent of carbon. This molten high-carbon low-chromium alloy is thentransferred to a converter and side-blown with pure oxygen or enrichedair to reduce its carbon content below 0.2 percent, as described in thepreceding example.

A preferred procedure which we have successfully employed in theproduction of rustless iron involves the dilution of molten highcarbonferrochromium with molten steel to initially reduce the carbon contentof the resulting bath, the latter being tapped directly from the furnacein which it is produced and run into the side-blown converter containingthe molten steel, where Bessemerization with oxygen or enriched air asalready described further reduces the carbon content to a value below0.2%. This conserves heat, reduces manipulation, and saves lossesincidental to remelting.

Following is a specific example of our procedure in producing an ironchromium alloy of the rustless iron type. Steel scrap and. l

high-carbon ferrochromiumwere melted in an electric furnace inproportions to give an alloy of chromium content appropriate for theproduction of rustless iron by this process. A sample of the alloy takenat the end of this electric furnace heat analyzed 20.66% chromium, 1.24%carbon, and 0.14% silicon. This molten alloy from the electric furnacewas rapidly transferred to a basic-lined, side blown converter. In thiscase commercially pure oxygen served as the blast and the total blowingtime was 5% minutes; Before the addition of deoxidizers, the alloycontained 17.71% chromium and 0.08% carbon. The ingots cast therefrom.contained 17.74%

- creased blast pressures are' desirable for blasts. of low oxygencontent and at the startof all.

Ill

chromium, 0.10% carbon, and 0.24% silicon.

The average physical properties of specimens of the product obtained bythe procedure described in the preceding paragraph were found to be:yield point, 58600 lbs; ultimate strength, 86600 lbs. elongation, 23%;and reduction in area, 51.7%. In an electric furnace finished productsimilarly produced and analyzing 15.75% chromium, 0.12% carbon, 0.32%silicon, the following physical prcperties were ascertained: yield oint,61300 lbs.; ultimate strength, 87250 bs.; elongation, 27% and reductionin area, 67.5%.

As a result of our extensive investigations in which blasts with pureoxygen and air enriched with Various proportions of oxygen have beenemployed, it has been shown that the temperature of the metal increasesrapidly as the oxygen content of the blast increases, the time necessaryfor a given reduction of carbon content diminishes, and the slag thataccumulates on the surface of the metal decreases in volume,each ofthese features contributing materially to the feasibility and efliciencyof the process. Experience has shown also that it is advantageous tohave the blast applied adjacent to'the surface of the bath in theconverter,preferably sothat it impinges directly onto the surface of themetal, in order that the oxygen of the blast may effectively burn outthe carbon contained in the metal; and furthermore, that inside-blownheats. The use of higher percentages of oxygen in the blast increasesthe velocity of the exothermic reactions which results in diminishedchromium losses through shortening the time during which the metal issubjected to the action of the oxygen. A criterion of the extent of thecarbon removal has been discovered in the flame drop. This drop in thelong, bright flame issuing from the converter indicates that the carbonin the alloy has been reduced to about one-quarter of one percent, inthe case of alloys having the chromium content of rustless irons. Theadditional time of blowing re uired is a function of the carbon contentdeslred in the alloy and of the oxygen content of the blast.

We find our improved process especially advantageous for making rustlessiron alloys, since our investigations have proven that, at the electricfurnace temperature employed (upwards of 1600 C.) a substantially higherchromium to carbon ratio (consistent with a given chromium recovery) canbe obtained in the rustless iron so produced than heretofore. 'We areunable to state the maximum temperature attained in our process duringthe blow but believe it to be considerably above 1600 C. Those skilledin this art will, of course, recognize the practical impossibility ofmeasuring these temperatures under the conditions existing in theconverter.

molten alloy is held at such temperatures during the blow as to bringabout a greater aflinity between oxygen and carbon than between underthese conditions the carbon may be largely or substantially eliminatedby oxidation before any considerable percentage of chromium is removed.At the blowing temoxygen and chromium, it being stated that perature weuse, such conditions do not exist but, on ,the contrary, chromium is infact Strongly oxidized throughout the blow, that is to say, many poundsof chromium oxidize for each pound of carbon eliminated, especiallytoward the end of the blow; and the deterioration of the converterlining proceeds very rapidlyand becomes prohibitive in bottom blowing.However, we have discovered that the temperature we employ ispracticable and decidedly economical because, by proper control ofthistemperature and preferably by side blowing with oxygen enriched air orsubstantiallypure oxygen, the removal of carbon is so greatly expeditedthat a carbon content p as low as about 0.02 per cent may be attainedeconomicall and the total chromium losses are materially reduced,although the rate of chromiumoxidation or pounds of chromium oxidizedper unit of time appears to be increased. In other words, while thehigher concentration of oxygen may expedite the oxidation of chromium aswell as carbon, the

carbon content is'reduced to the desired per-,

centage so much quicker than heretofore that the total quantity ofchromium lost during the blow is greatly diminished. Moreover,

the blowing period is so much shortened that the oxygen of the blast iseconomically utilized, and any objectionable attack on the converterlining is materially reduced, bringing our procedure well withinpractical and economical operating requirements We claim: a

1. The process of making a low-carbon iron-chromium alloy'of therustless iron type which comprises preparing a molten bath consistinessentially of-iron, chromium and carbon, t e carbon being present in asubstantial proportion not exceeding 2.0% and the chromium contentcorresponding to that of rustless iron, side-blowing the bath with a 7blast containing from 50% to 100% of free oxygen whereby oxidationof'constituents of the bath including carbon and a minor por tion onlyof the chromium takes place and the temperature of the bath is raised toand I maintained at above 1600 C. solely by means of the exothermicreaction induced b the blowing operation, and continuing the lowinguntil the carbon content of the bath is reduced to not more than 0.2%. i

2. The process of making a low carbon iron-chromium alloy of therustless iron type which comprises preparing a molten bath consistingessentially of iron, chromium and carbon, the carbon being present in asubstantial proportion not exceeding 2.0% and. the chromium contentcorresponding to that of restless iron, side-blowing the bath with e,blast of commercially pore oxy en whereby oxidation of constituents ofthe ath including carbon and 2* minor portion only of the chromium takesplace and the temperature of the bath is raised to and maintained atabove 1600 C, solely by means of the exothermic reaction induced .by theblowing operation, and continuing the blowing until the carbon contentof the bath is reduced to s not more than 0.2%.

In testimony whereof, we afix our signetures.

FREDERICK M. BECKET. JMES H. GRETCHEN.

